Eric’s right doing it in AE is a PITA. I had to do something similar for a digital series once though. This is from a long time ago so it might not be the most efficient way but it worked for me. This way your don’t have to use a source at rect expression and its all done on one layer.
On your text layer do the following:
Fast box blur until the letters are pretty blurry, Add a fill effect to fill your background color.
Add a curves effect and go to the alpha channel and crank it all the way up. Now you should see the entire bounding box of the text layer is solid and filled to the color in the fill effect.
Now add a CC composite effect and set the composite original to In Front. That should put your original text in front of the filled area.
You can animate it on however you need and the fill should respond. I used linear wipe.